By use of genetic, biophysical and chenical methods determine the type of genetic alteration(s), function and/or structu re(s) altered in the influenza temperature-sensitive mutants and recombinants furnished by the Government. This will include (a) composite properties of the test viruses (b) identification of viral gene functions (c) multiple factor crosses for genetic analysis (d) tests for genetic stabiltiy in vitro and in vivo (e) tests for the capacity to establish persistent nonproductive infections.